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Author Topic: Suction Filtration in Organic Chemistry Labs  (Read 3491 times)

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Sonia

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Suction Filtration in Organic Chemistry Labs
« on: September 26, 2011, 06:26:53 PM »

Hi there,

I am kind of having a hard time in figuring out these questions. Any help would be highly appreciated.

1) When rinsing crystals that have been collected by suction filtration, you should break the vacuum and gently loosen the crystals before rinsing. Why?

2) In a suction filtration apparatus a thick heavy-walled red rubber tubing is used. Why?

3) When breaking the vacuum in a suction filtration apparatus you should break the vacuum by lifting up the funnel and/or the filtervac then turn off the water. Why?
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Arkcon

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Re: Suction Filtration in Organic Chemistry Labs
« Reply #1 on: September 27, 2011, 12:55:47 AM »

Your textbook or lab manual or class notes (your own or someone else's) may have the exact answer your instructor is looking for, so you should check harder to give the perfect answer.  However ...

for 1).  what can you guess would happen if you break vacuum some other way?

for 2).  try to realize the world of chemistry is not in your lab, I have never used the "thick heavy-walled red rubber tubing".  However, have you ever tried to use thin rubber tubing for this application?  What happens?

for 3).  you should have noticed a theme with these questions by now.  What are the other two questions trying to prevent?

Really develop your text book, lab manual, and class notes reading skills.  You are missing some very basic steps.  If you've missed these, you have likely missed some more important steps, and you could get hurt in the lab, not just a low grade from missing points.
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Sonia

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Re: Suction Filtration in Organic Chemistry Labs
« Reply #2 on: September 27, 2011, 07:58:34 AM »

I read through the lab manual very thoroughly and this is what I found:

for 1) When rinsing crystals that have been collected by suction filtration, you should break the vacuum and gently loosen the crystals before rinsing because after filtration there will remain a residual layer of "dirty" mother liquor on the crystals of acetanilide that must be washed off using clean water.

for 2) In a suction filtration apparatus a thick heavy-walled red rubber tubing is used because this will greatly reduce the chance of accidently breaking one of the expensive flasks.

for 3) When breaking the vacuum in a suction filtration apparatus you should break the vacuum by lifting up the funnel and/or the filtervac then turn off the water because while filtering, pressing the funnel down forms an air-tight seal between the funnel and/or the filtervac and flask. Turning on the water tap gives you maximum flow and thus maximum suction. So therefore, breaking the vacuum would break the air-tight seal and after that turning off the water would minimize the suction.

Please let me know if I am on the right track. Thanks a bunch!

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fledarmus

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Re: Suction Filtration in Organic Chemistry Labs
« Reply #3 on: September 27, 2011, 08:25:13 AM »

1) I would think this is good enough. You might want to consider why breaking the vacuum and loosening up the crystals would be more effective than just pouring the water into the tightly packed crystal mass with the vacuum still on.

2) How does the type of tubing affect the chances of breaking the glassware? I think you're on the wrong track here. Think about trying to suck a thick milkshake through a straw - what happens if you use a really thin, flimsy little straw to try to suck up the milkshake? How about a larger straw with stronger sides?

3) Do you know how a vacuum aspirator works? Here is a cross section (sideways to the one in your sink)

from Wikipedia, and the description is http://en.wikipedia.org/wiki/Eductor-jet_pump. Basically, it works because the water is being forced under high pressure through the "motive fluid nozzle", creating a vacuum which draws the air from your flask through the "inlet gas" opening. If you shut off the flow of water without breaking the vacuum, what would happen to the water in the aspirator? Keep in mind that you now have a vacuum in your flask...
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